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the EPA's National Air Toxics Assessment, results in a should be able to address the identified information gaps and
much less significant role for formaldehyde. substantially decrease the uncertainty associated with airport-
Table 2 lists additional HAPs identified by this analysis related HAPs. More detail is presented in Section 9.
that may also be important to consider for further evalua-
tion. These HAPs have not undergone a formal toxicity Research Item 1: Identify the effects of ambient condi-
evaluation. A comparison with structurally similar HAPs, tions (temperature, pressure, humidity) and engine technol-
however, indicates they may be important in terms of ogy on HAP emissions at various idle/taxi power settings.
relative toxicity and emissions. Note that glyoxal and This project would measure the emission rates of various
methylglyoxal are currently not classified by the EPA as HAPs compounds from commercial aircraft as a function of
HAPs, but are included on the EPA's "Master List of Com- thrust level near idle and as a function of environmental vari-
pounds Emitted by Mobile Sources." This analysis indicates ables such as temperature, humidity, and pressure. The goal
they may be among the most important airport HAPs. of this project would be to improve our quantitative un-
3. The two gas-phase HAPs for which non-aircraft sources derstanding of the largest airport-related HAPs emission
(GSE, GAV, and stationary sources) are most important, source--jet engines operating at low power. Emissions data
when viewed in the same type of emissions-toxicity for commercial aircraft are only available in the narrow tem-
weighting, are benzene and 1,3-butadiene, both of which perature range (8° to 35°C, 46° to 95°F), and show that HAP
are primarily emitted by gasoline engines (as opposed to emissions increase greatly with decreasing temperatures. The
those that run on diesel or compressed natural gas). lack of knowledge regarding the temperature dependence of
4. Speciation profiles for aircraft-emitted HAPs that were HAP emissions, and how it depends on engine technology,
based on the work of Spicer, Holdren et al. (1994) are ac- currently results in uncertainties of more than a factor of 2 in
curate, as the relative speciation of HAPs in aircraft exhaust modeled HAP concentrations and risk. This is especially im-
has been found to be constant among numerous types portant for airports located in cold environments.
of engines subsequently characterized (see Section 5.1).
Research Item 2: Quantification of the "real-world"
Recent field measurements (EXCAVATE, APEX 1, 2, 3)
thrust values used at airports during the idle phase.
have greatly expanded the knowledge base regarding the
This project would investigate and quantify the actual
quantification and speciation of aircraft HAPs emissions.
thrust values used by commercial aircraft at several airports.
Recent research indicates that the standard power setting (7%
rated thrust) that is used to calculate emission inventories
1.6 Prioritized Research Agenda
does not represent actual thrust levels used during aircraft
Based on the results of this research, the following areas of taxi and idle. This introduces large uncertainties in emission
research have been identified that would be most beneficial in inventories and risk assessment, since HAP emissions are very
addressing these information gaps. The projects listed below sensitive to small changes in engine power.
Table 2. Aviation-related HAPs of potential concern.
HAP Basis for Concern
Crotonaldehyde Crotonaldehyde is structurally similar to the
highly reactive compound acrolein, and its
airport emissions may be comparable to those
of benzene and 1,3-butadiene.
Glyoxal Glyoxal is a mutagenic aldehyde with two
carbonyl groups, and has been shown to act as
a tumor promoter in rats. Airport emissions of
glyoxal are comparable to those of benzene
and 1,3-butadiene.a
Methylglyoxal Methylglyoxal is a mutagenic aldehyde with two
carbonyl groups that has a DNA adduct
formation potency 20-fold greater than
acetaldehyde. Airport emissions of
methylglyoxal are comparable to those of
benzene and 1,3-butadiene.b
Propanal (propionaldehyde) Propanal (propionaldehyde) is structurally
similar to acetaldehyde and its emissions are
comparable to naphthalene.
a
IARC 1991; NEG 1995.
b
IARC 1991; Vaca, Nilsson et al. 1998.
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Research Item 3: Characterization of HAP emissions affect potential receptors (e.g., nearby residents, airport-
from general aviation (e.g., piston engine aircraft). based workers, passengers). This should be done using
This project would support emissions measurements of dispersion/chemistry models tested against complementary
general aviation aircraft. HAP emissions (with the exception measurements. Conclusive deduction of source apportion-
of lead) from piston engine aircraft, turbojet engines, and ment will require coordinated measurements of the main
low-bypass turbofan engines (such as business jets) are combustion gases (CO2, CO, NO, NO2), speciated HAPs (e.g.,
largely unknown and should be quantified. This is most benzene, formaldehyde, etc.) and particulate matter (PM)
important at general aviation airports. (characterized by number/size, mass, and chemical composi-
tion). Source apportionment is required to evaluate proposed
Research Item 4: Identify the emission sources most im- emissions mitigation strategies. This project will help airport
portant to on-airport and off-airport exposure. operators to identify the "low-hanging fruit" with regards to
The purpose of this project is to identify which emission minimizing the health risk presented by the various emission
sources (aircraft, GSE, terminal traffic, etc.) most greatly sources present at an airport.
